//```// YF YUSUF
#define YF ios_base::sync_with_stdio(0);cout.setf(ios::fixed);
#define YUSUF cout.precision(0);cout.tie(0);cin.tie(0);
// #include <bits/stdc++.h>
#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <unordered_map>
#include <cmath>
#include <numeric>
#include <queue>
#include <stack>
#include <cassert>
#include <climits>
#include <string>
#include <iomanip>
#include <cstdlib>
using namespace std;
#ifdef YF_CHECK
bool Output=1;
// #include "g/debug.h"
// #include "g/bigint.h"
// #include "g/text.h"
#else
#pragma GCC optimize ("unroll-loops")
#pragma GCC optimize ("inline")
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("O3")
bool Output=0;
#define deb(x...) 42
#endif
#ifdef DEV
bool LOUT=1;
#else
bool LOUT=0;
#endif
using ll = long long;
using ld = long double;
using sll = set <ll>;
using vll = vector <ll>;
using mll = map <ll,ll>;
using pll = pair <ll,ll>;
using vvl = vector <vll>;
using vpll = vector <pll>;
template<class T>T MIN(T&a,T b){a=min(a,b);return a;}
template<class T>T MAX(T&a,T b){a=max(a,b);return a;}
#define revers(a) reverse(all(a))
#define all(a) a.begin(),a.end()
#define sortt(a) sort(all(a))
#define sgr v+v+1,(tl+tr)/2+1,tr
#define sgl v+v,tl,(tl+tr)/2
#define pb push_back
#define ins insert
#define S second
#define F first
ll BP(ll a,ll b,ll mod=1e9+7){
if(b==0)return 1;
ll q=BP(a,b/2,mod);
return ((q*q)%mod*(b%2?a:1ll))%mod;
}
ll f(ll x){return x*(x+1)/2;}
ll dup(ll a,ll b){return (a+b-1)/b;}
ll lcm(ll a,ll b){return a/__gcd(a,b)*b;}
ll invf(ll x){return (-1+sqrt(1+8*x))/2;}
const ll mod=998244353;
const ll INF=1e18;
const ll inf=1e9+7;
const ll N =2e5+7;
vpll g[N];
void YF_MAIN(ll TEST){
ll n,m;
cin>>n>>m;
vpll v(m);
for(auto &[b, p] : v)cin>>b>>p;
for(int i=0;i<m;i++){
for(int j=i+1;j<m;j++){
auto [b, p] = v[i];
auto [B, P] = v[j];
ll c = abs(b-B);
if(c%p==0)g[i].pb({j, c/p});
if(c%P==0)g[j].pb({i, c/P});
}
}
ll s=0,f=1;
priority_queue <pll,vector<pll>,greater<pll> > q;
vector<ll> d(m+1,INF);
d[s]=0;
q.push({0,s});
while(q.size()){
auto [dist,v] = q.top();
q.pop();
for(auto [to,dto] : g[v]){
if(d[to] > d[v] + dto){
d[to]=d[v]+dto;
q.push({d[to],to});
}
}
}
cout<<(d[1]==INF ? -1 : d[1]);
}
bool TECT=0;
bool CASE=0;
bool OUTP=1;
const ll LN=2e0 + 7; ll lg[LN];
const ll SN=2e0 + 7; ll SM[SN];
const ll FN=2e0 + 7;
ll FACT[FN],inv[FN],FMOD=inf;
ll PER(ll n,ll k){return FACT[n] *inv[n-k]%FMOD;}
ll CNK(ll n,ll k){return PER(n,k)*inv[k ]%FMOD;}
void BEFORE(){
for(ll i=2;i<SN;i++){
if(SM[i])continue;
for(ll j=i;j<SN;j+=i)
MAX(SM[j],i);
}
for(int i=2;i<LN;i++) lg[i]=lg[i/2]+1;
FACT[0]=inv[0]=1;
for(int i=1;i<FN;i++){
FACT[i]=FACT[i-1]*i%FMOD;
if(i<5e5+7)inv[i]=BP(FACT[i],FMOD-2,FMOD);
}
}
signed main(){
if(OUTP){YF YUSUF}
// freopen((".in"),"r",stdin);freopen((".out"),"w",stdout);
cout<<(LOUT && Output ? "\nYF_OUTPUT:\n\n" : "");
int TEST=1;
if(TECT)
cin>>TEST;
BEFORE();
for(int T=1;T<=TEST;T++){
if(CASE)
cout<<"Case "<<T<<": ";
YF_MAIN(T);
cout<<(T==TEST ? "" : "\n");
}
return 0;
}
//```
